Increased male sex ratio among brachypterous progeny in Melittobia femorata, a sib-mating parasitoid wasp (Hymenoptera: Eulophidae)

Abstract

Abstract Melittobia are gregarious ectoparasitoid wasps that primarily attack various solitary bees and wasps. Characterized by high levels of inbreeding and an extremely female-biased sex ratio, these wasps appear to satisfy Hamilton's criteria for local mate competition. However, previous studies of sex ratio have failed to take into account an important aspect of Melittobia life history, namely that every clutch represents the combined reproductive output of the initial foundress female plus as many as 37 non-disperser short-winged daughters. Melittobia femorata Dahms is unique among the 13 species of Melittobia in that adults emerge as two temporally distinct clutches. While the overall sex ratio of the combined progeny from both clutches (0.025 ± 0.01) is typical for that for other Melittobia species (between 0.02 - 0.04 for single foundress cultures of the five other species included in this study), the sex ratio for the brachypterous first clutch of M. femorata from field-parasitized hosts averaged about 10 times greater (0.303 ± 0.10). Laboratory experiments with single foundress M. femorata cultures on the same host species (Trypoxylon politum Say) maintained at 25°C or 30°C produced smaller first clutch sizes compared to the field-infested hosts. While the number of brachypterous first clutch daughters was similar, significantly reduced first clutch sex ratios relative to field-parasitized hosts (0.07 - 0.10) were due to significantly fewer males being produced. Possible reasons for these differences and the elevated first clutch sex ratio in this species are discussed.